The invention relates to a wheeled undercarriage for mobile ladder stands and platforms. In particular, the undercarriage has four retractable wheels that are positioned specifically at the rear and at the middle of the undercarriage, thereby providing enhanced mobility for mobile ladder stands and platforms.
Mobile ladder stands are used by workers and others who need to reach products or materials stored on shelves in stores and warehouses. Mobile ladder stands typically have one or two pairs of support legs mounted at the front and/or rear of the frame undercarriage, as well as two or three pairs of wheels mounted to the undercarriage frame. In a conventional four-wheeled, mobile ladder stand, a pair of retractable wheels is located towards the front of the ladder stand adjacent a front pair of legs. Another pair of retractable wheels is mounted at the rear of the frame undercarriage near a rear pair of legs. An actuation mechanism is provided to extend and retract the front and the rear wheels in unison. When the wheels are extended, the mobile ladder stand (or mobile platform) is supported on the ground surface by the extended wheels, and the mobile ladder stand can be manually pushed or rolled in order to position the ladder stand for use. When the wheels are retracted, the ladder stand is supported on the ground surface by foot pads at the bottom of the front and rear legs, thereby preventing the ladder stand from rolling. In a conventional four-wheeled ladder stand, the rear wheels are fixed and the front wheels (i.e., the wheels located near the lowermost stair tread) are swivel casters. This arrangement allows the front swivel casters to track the straight path of the rear wheels when the ladder stand is pushed down an aisle, but the front swivel casters also allow the ladder stand to be maneuvered around corners or the like. As mentioned, these ladder stands typically have actuation mechanisms to extend and retract the wheels. However, some four-wheeled ladder stands do not include the rear pair of legs and footpads. In this configuration, the rear pair of wheels are not retractable and remain in contact with the ground surface when the ladder stand is in both a mobile position and a non-mobile position. When the front wheels are retracted, a ladder stand with this configuration is supported on the ground surface in a non-mobile, fixed position by the front footpads and the rear wheels. The rear wheels in this configuration are not swivel casters.
Typically, the actuation mechanism includes either a foot lever or a hand lever to extend the wheels and lift the ladder stand (or mobile platform) off of the footpads at the bottom of the legs. The actuation mechanism typically includes some sort of locking mechanism that locks the wheels in the extended position until the weight of the user manually actuates a release lever. In a mobile ladder stand, the release lever normally responds to the weight of the user from the user""s foot before the user steps onto the ladder stand. For example, the release lever can consist of a foot bar near the lowermost stair tread, or the lowermost stair tread can itself be made pivotable and, in fact, act as the release lever.
Large mobile ladder stands (or large mobile platforms) become awkward to maneuver in small spaces because the turning radius increases as the length of the undercarriage increases. This problem has been addressed in the industry by the provision of six-wheeled ladder stands such as disclosed in Gillis, et al., U.S. Pat. No. 6,039,147 and U.S. Pat. No. 5,941,341, both entitled xe2x80x9cMobile Ladder Stand.xe2x80x9d These patents disclose a mobile ladder stand that has a front pair of wheels mounted to the front of the ladder base beneath the top of the first stair tread, a center pair of wheels mounted to a center beam of the ladder base, and a rear pair of wheels mounted to the rear of the ladder base. A set of legs and footpads are mounted to the front of the ladder base. The front and center wheels are retractable. The front pair of wheels are swivel casters. The diameter of the center pair of wheels is preferably larger than the diameter of the front and rear pair of wheels. When the front and center wheels are extended, the ladder is supported on the ground surface by the front and center wheels, and the rear wheels and the front support feet are not generally in contact with the ground surface. In this position (i.e. the mobile position), the ladder stand can be rolled and the turning radius of the ladder is defined generally by the distance between the center pair of wheels and the front pair of swivel casters. When the front and center wheels are retracted (i.e. non-mobile position), the ladder stand is supported on the ground surface by the front footpads and the rear wheels, and rolling is prevented. Although it is not preferred, the Gillis, et al. patents suggest that the rear wheels can be replaced by a pair of foot pads at the rear of the base.
An object of the invention is to provide another undercarriage for a mobile ladder stand or other mobile warehouse apparatus (e.g., mobile platform), in which the undercarriage is actuated between a mobile and non-mobile position, and provides a turning radius and maneuverability similar to the aforementioned six-wheeled ladder stand disclosed in Gillis, et al. U.S. Pat. Nos. 6,039,147 and 5,941,341.
The invention is an undercarriage for a mobile ladder stand or other similar warehouse apparatus that includes a pair of retractable wheels located at the rear of the undercarriage frame and a pair of retractable wheels located in the middle of the undercarriage frame. The undercarriage includes a front pair of legs and footpads located at the front of the undercarriage frame and a rear pair of legs and footpads located at the rear of the undercarriage frame. The rear pair of retractable wheels are swivel casters, and the middle pair of retractable wheels are fixed. There are no wheels at the front of the undercarriage frame. The undercarriage also includes an actuation mechanism that extends and retracts the middle and rear wheel sets. When the middle and rear wheels are extended (i.e., the mobile position), the ladder stand (or mobile platform) is supported on the ground surface by the middle and rear wheel sets thus enabling the ladder (or mobile platform) to be rolled along the ground surface. When the wheels are extended and the undercarriage is in the mobile position, the turning radius is generally defined by the distance between the middle set of wheels and the rear set of swivel casters, thereby rendering the undercarriage more easily maneuvered than other commercially available four-wheeled, mobile ladder stands.
The middle set of wheels and the rear set of swivel casters are preferably extended and retracted in unison. When the middle pair of wheels and the rear pair of wheels are retracted, the undercarriage is supported on the ground surface by the pair of footpads at the front of the undercarriage frame and the other pair of footpads at the rear of the carriage frame. This provides an especially sturdy support when the ladder stand (or mobile platform) is in the non-mobile position.
The preferred actuation mechanism includes a front pivotable lever connected to a foot pedal located near the front of the undercarriage frame. The foot pedal is actuated by the user in order to extend the wheels and lift the front and rear footpads off the ground surface. The front pivotable lever connected to the foot pedal is also connected by a mechanical linkage to a middle pivotable lever connected to the middle retractable wheel set and a rear pivotable lever connected to the rear retractable wheel set. Preferably, a connecting rod connects directly to the front pivotable lever, the middle pivotable lever and the rear pivotable lever in order to facilitate the extension and retraction of the wheels in unison. A spring-loaded locking mechanism preferably locks the actuation mechanism when the wheels are in the extended position in order to allow the mobile ladder stand (or mobile platform) to be rolled over the ground surface. A release lever actuated by a foot bar or a pivoting lowermost stair tread is provided. When the user steps on the release lever with their foot, the combined weight of the ladder and user causes the extended wheels to retract and push the actuation mechanism against the force of the spring. This allows the undercarriage to rest on the front and rear foot pads in the non-mobile position.
Preferably, the diameter of the middle pair of retractable wheels is larger, for example about five inches, than the diameter of the rear wheel set, for example about four inches. With this configuration, it is easy to tilt the undercarriage rearward when the wheels are extended so that the undercarriage rides on the middle and rear pairs of wheels with sufficient clearance for the front foot pads above the ground surface.
It should be apparent to those skilled in the art that a four-wheeled, mobile ladder stand constructed in accordance with the preferred embodiment of the invention allows the user to push the ladder stand efficiently in a straight direction, and also allows the user to turn the mobile ladder stand around a corner by pivoting about the center wheels and allowing the swivel casters of the rear wheel set to turn in the direction of the corner. In this matter, a four-wheeled mobile ladder stand (or mobile platform) constructed in accordance with the invention is able to turn around tight corners (for example, 90xc2x0 corners), with only approximately xc2xd of the turning radius of a normal, four-wheeled ladder stand.